1. Field of the Invention
This invention relates generally to semi-conductive silicone rubber compositions and, more particularly, to a semi-conductive silicone rubber composition which exhibits electric conductivity stable over a broad range of aspects.
2. Description of the Prior Art
Conventionally, silicone rubber compositions, for cure into silicone rubber, having a volume resistance value of approximately 105 to 1012 xcexa9xc2x7cm as an electric conductivity in the semi-conductive region have being broadly utilized for the applications of ranging from transfer rollers used as one members in image forming devices for electrophotographic apparatuses over to a variety of OA equipment and electronic components, as can be known from Japanese Patent Laid-open No. H10-254215 laid open on Sep. 25, 1998.
These semi-conductive silicone rubber compositions, in general, are given electric conductivity by adding insulating silicone rubber with conductive carbon black. For example, Japanese Patent Laid-open No. S54-139659 (Oct. 30, 1979) describes a conductive organopolysiloxane elastomer using both furnace black and acetylene black.
However, the semi-conductive silicone rubber composition of this kind blended and dispersed with carbon black is ready to greatly vary in its resistance value within the semi-conductive region due to various factors, thus suffering from a problem with insufficient resistance stability and reproducibility. Such resistance variation principally results from the following factors:
(a) environmental change in temperature, humidity, etc.,
(b) change in blending amount of carbon black,
(c) aging during applying a constant high voltage, and
(d) voltage change during applying a high voltage.
That is, the problem (a) is to be encountered in an ion-conductive type rubber composition blended with conductive oil or plasticizer in addition to a small amount of conductive carbon black. That is, silicone rubber is impaired in its inherent environmental characteristics and made dependent of electric resistance upon environmental change, such as humidity or temperature. As a result, the resistance largely decreases at an elevated temperature and humidity. The problem (b) may arise in a rubber composition added with carbon black, such as furnace black of FEF, GPF or the like, acetylene black or Ketjen black for providing electric conductivity. That is, slight change of addition amount causes large resistance variation and hence makes it difficult to put electric resistance under control. In the problem (c), where a constant high voltage is continuously applied to a rubber composition added with carbon black similar to that of (b), the resistance value greatly decreases during the application of voltage. In the problem (d), when varying high voltage is applied to a rubber composition added with carbon black similar to that of (b), resistance value largely varies with variation of voltage (or the resistance value largely decreases with increase of voltage). This results in over-current flow and difficulty in controlling current.
Where a transfer roller as above is formed using a semi-conductive silicone rubber composition having such a resistance value as ready to largely vary due to the above factors, there is a necessity of using an accurate application-voltage control device to put required transfer current under control. This, however, incurs a problem with apparatus complexity or cost rise. In addition to the problem with resistance variation, there is also restriction that carbon black in a comparatively small amount only can be added in view of avoiding rubber from hardening as well as from worsening in operationality or workability. Thus, there has been inconvenience in providing resistance broadly within the range while adjusting carbon amount to meet the product application.
Incidentally, the above Japanese Patent Laid-open No. H10-254215 discloses a technology to stabilize the resistance value by dispersing two kinds of carbon black different in oil absorption characteristic (specifically, ketjen black and thermal black) in three kinds of rubber different in solubility parameters (specifically, nitrile-butadiene rubber, ethylene-propylene-dien rubber and chloroprene rubber or styrene-butadiene rubber).
Therefore, it is a primary object of the present invention to provide a novel semi-conductive silicone rubber composition.
Another object of the invention is to provide a semi-conductive silicone rubber composition having resistance value stable within the semi-conductive region.
The present inventors have eagerly made many studies and reached a completion of the present invention through the following finding. That is, a certain kind of soft carbon, although conventionally used exclusively as reinforcer or filler for less-hard rubber without considered as carbon black for giving conductivity, is surprisingly capable of providing a suitable range of conductivity to silicone rubber in a manner free from adverse effects upon formability, wherein a silicone rubber composition blended therewith is less dependent upon the changes of various factors including carbon blending amount and hence having a resistance value stable in various aspects, thus solving all the above-stated problems at a same time.
That is, the present invention has a structure given below as means to solve the problem.
(1) A semi-conductive silicone rubber composition contains carbon black, wherein the carbon black includes thermal black obtained by thermally cracking a natural gas and having a specific surface areaxe2x80x94nitrogen absorption method of 8.0 to 10.0 m2/g, a dibutyl phthalate absorption number of 30 to 40 cm3/100 g and an averaged particle size of 200 to 330 nm.
(2) In the composition of (1), silicone rubber is heat cured rubber based on polyorganosiloxane.
(3) A rubber roll comprises: a conductive core member; and a silicone rubber layer coated on an outer peripheral surface of the conductive core member, the silicone rubber layer comprising a semi-conductive silicone rubber composition containing carbon black, the carbon black including thermal black obtained by thermally cracking a natural gas and having a specific surface areaxe2x80x94nitrogen absorption method of 8.0 to 10.0 m2/g, a dibutyl phthalate absorption number of 30 to 40 cm3/100 g and an averaged particle size of 200 to 330 nm.
(4) In the composition of (3), silicone rubber is heat cured rubber based on polyorganosiloxane.
The semi-conductive silicone rubber composition is basically a polyorganosiloxane composition which is curable, at normal temperature or by heating, into a rubber resilient material and added with thermal black at least satisfying the above conditions. Various additives and the like may be blended as required.
The thermal black employed in the invention is carbon black produced by a thermal (or thermal cracking) method, i.e. thermally cracking a natural gas introduced in a furnace heated above a thermal-cracking temperature by burning fuel. The carbon black has large particle size, low structure and small specific surface area as compared to other furnace black, thus providing a merit of reduced impurity owing to the complete combustion method. The specific surface areaxe2x80x94nitrogen absorption method must be within a range of 8.0 to 10.0 m2/g, a dibutyl phthalate absorption number be 30 to 40 cm3/100 g and an averaged particle size of 200 to 330 nm (hereinafter, the thermal black fallen within this range is referred to as MT carbon).
MT carbon is capable of providing silicone rubber with proper conductivity, against the generally accepted concept that the carbon black having a large particle size and low structure is improper as a conductivity-applying agent. As a reason of this, it is to be considered that particles of MT carbon be enhanced in degree of contact with one another because silicone rubber is by far softer than other rubber.
MT carbon includes N991 (product name, produced by Cancarb), N908-UP (product name, produced by Cancarb), LPT (product name, produced by Cancarb) and Arosperse (product name, produced by Engineered Carbons).
The polyorganosiloxane base polymer, as a principal ingredient of the semi-conductive silicone rubber composition of the invention, suitably employs heat cured rubber, such as dimethyl siloxane and phenyl siloxane. One or two or more in combination of these of silicone raw rubber may be used. The polyorganosiloxane polymer is added with MT carbon as an essential ingredient. Besides, various additives as arbitrary ingredients are to be blended and dispersed homogeneously as required which include curing agents, such as organic peroxide vulcanization agent and addition cross-linking agent, silica reinforcing filler, azodicarbonaamide or isobuthynitrile-azide blowing agent, other heat-resisting antioxidant, workability auxiliaries and so on.
The semi-conductive silicone rubber composition of the invention has a resistance value moderately varying with variation of carbon addition amount. For example, the change of resistance value from 1012 xcexa9 to 104 xcexa9 requires an increase an MT carbon amount by nearly 25 weight part. Consequently, in a semi-conductive region where resistance value abruptly varies with variation in carbon-black addition amount, the resistance value is less affected and varied even if there is slight difference in carbon black content caused due to loss upon kneading or is delicate difference in dispersion caused upon forming such as by extrusion. As a result, the silicone rubber composition is high in resistance-value reproducibility and excellent in mass-production stability thus being utilized for various applications of semi-conductive rubber products. Meanwhile, there is an advantage that the resistance value is almost stable when applying a constant high voltage for a long time.
On the other hand, the semi-conductive silicone rubber composition of the invention exhibits preferred stability of resistance value against variation in application voltage. For example, provided that a current value upon applying 1000V is I1 and a current value upon applying 2000V is I2, then I2/2I1 lies within a range of 1.2 to 1.5. This range represents a current value available almost in proportion to application voltage. Thus, dependency on voltage is extremely low.
The semi-conductive silicone rubber composition can make use of the characteristics that MT carbon is to be blended much in amount in a manner free from worsening operationality or workability. By adjusting the blending amount, to what degree conductivity is given is arbitrarily selectable from a broad range. The blending amount of MT carbon is not especially limited to but preferably selected within a range of 30 to 100 weight part for 100 weight part of silicone rubber in a manner depending upon the application. The invention is not an ion conductive type rubber composition and does not contain conductive oil or plasticizer at all. Rather, the invention is given conductivity by the addition of MT carbon singly. Consequently, resistance value can be held stable in a manner free from bad affection on the environmental characteristics inherently possessed by silicon rubber and from being affected by environmental change in temperature or humidity.
Incidentally, where using carbon black fallen outside the range in any of values of specific surface areaxe2x80x94nitrogen absorption method, dibutyl phthalate absorption number and averaged particle size or the carbon black obtained by other process than thermally cracking a natural gas, it is impossible to obtain stable resistance value or desired dependency on voltage within the above-mentioned middle-resistance range. Moreover, favorable operationality is not to be provided during blending in great amount.
According to the semi-conductive silicone rubber composition of the present invention, the stability of resistance value in a semi-conductive region can be greatly improved in broad aspects while keeping the excellent environmental characteristic of silicone rubber stable in electric resistance against environmental change. Accordingly, the semi-conductive silicone rubber composition of the invention is well suited for the application for conductive members such as conductive rolls.
The above described objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.